Typical copolyester bottle resins employ polyethylene terephthalate (PET) and a comonomer, either a dicarboxylic acid such as isophthalic acid, or a diol such as 1,4-cyclohexane dimethanol or diethylene glycol. The comonomers are added to decrease the rate of crystallization of PET, in order to obtain clear preforms, which are stretch-blow molded into containers, such as soft drink bottles. If a crystallization retarding agent is not employed, crystallization of the preform occurs resulting in a hazy preform and a hazy container. However, if too much comonomer is used, the physical properties of copolyester resin are significantly weaker than PET resin.
Stress cracking can occur when the polyester container is under stress, such as that caused by carbonation of the beverage in a bottle. The weakest part of the bottle is the base where there is little stretching of the preform resulting in an essentially amorphous base section.
Stress cracking occurs over time, generally in the base of the bottle, causing a bottle under pressure with carbonated liquid to either lose pressure, or in the extreme, burst. Stress cracking can be initiated by the alkaline lubricants used in the bottle filling lines, or by the residues of alkaline cleaning solutions on store shelves.
To minimize caustic stress cracking (CSC) a high molecular weight copolyester is conventionally used; typically a bottle resin having an Intrinsic Viscosity (IV) greater than 0.82 dl/g. This high IV resin can be prepared by first melt polymerizing the copolyester to an IV in the range of about 0.5 to 0.65 dl/g followed by a solid state polymerization to raise the IV to above 0.80 dl/g. Newer polyester polymerization technologies enable an IV of 0.80 dl/g or higher to be achieved in the melt polymerization, without the need for the solid state polymerization process.
U.S. Pat. No. 4,755,404 to Collette is directed to refillable bottles. Based on the results from washing the bottles in a caustic soda solution, he found that all failures (leaks from bottles that had been pressurized with carbonated water) occurred in the unoriented base of the bottle. A bottle designed for refillable use must be recyclable a minimum of five times to be economical. Collette used polyester resins of different IV (0.72, 0.84 and 1.06 dl/g) and observed that the number of cycles to failure was 3, 6 and 7 respectively for bottles made from these different IV resins. This supports that high IV (greater than 0.80 dl/g) resins are required to achieve adequate resistance to caustic stress cracking.
U.S. Pat. No. 6,342,578 to Huang improved the CSC of bottles at high IV by introducing, at the end of polycondensation, one or more of unsubstituted anhydrides such as phthalic or succinic anhydride. The anhydrides reacted with the hydroxyl end groups to form carboxyl end groups (CEG). U.S. Pat. No. 7,087,706 to Caldwell disclosed the use of substituted cyclic anhydrides as other means to increase the CEG of the high IV resin. Both of these patents used high IV (0.80 dl/g or higher) for their examples, but neither taught whether the IV could be reduced at the higher CEG levels while maintaining satisfactory resistance to caustic stress cracking.